Things you must KNOW
You must KNOW your polyatomics
 You must KNOW how to write a
balanced formula
 You have to be able to balance a
chemical equation
 You need to know how to convert from
grams to moles and moles to grams.
 You need to be able to calculate molar
mass

What to expect in this chapter
You will be converting grams to moles
of one compound, then using the mole
ratio (balanced equation) go to moles
of another compound and then from
moles to grams in the new compound.
 You will learn how to identify a
limiting reactant, and how to calculate
percent yield.

What does the standard say and
what should I be able to do?
Sc2d. Identify and solve different
types of stoichiometry problems,
specifically relating moles to moles
Given the number of moles of a
reactant or product solve for the
moles of another reactant or product.
What is Stoichiometry?
 Stoichiometry: the study of quantitative
relationships between amounts of
reactants used and products formed by a
chemical reaction.
 Based on the Law of Conservation of
Mass
(mass reactants = mass products)
To prove Law of Conservation of Mass
1. Start with a balanced equation
2. Coefficient indicates the
#molecules/formula units and # of moles
3. Change moles to mass
4. Add up mass reactants and mass of products
 They should equal
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g)
What is Stoichiometry? :
continued
 Can use balanced chemical equation to get
mole ratios
2 Al + 3 Br2  2 AlBr3
2 mol Al
3 mol Br2
2 mol Al
2 mol AlBr3
3 mol Br2
2 mol AlBr3
 Find all the possible mole ratios for:
KClO3(s)  KCl(s) + O2(g)
Stoichiometric Calculations
 Use relationships to determine the amount of
product formed or reactant needed.
 Moles to moles
Moles known x mol unknown = mol unknown
mol known
from problem
ratio from balanced equation
Practice problem
How many moles of carbon
dioxide are produced when
10.0 moles propane (C3H8) are
burned in excess oxygen?
1.Write & balance the equation
Practice problem
How many moles of carbon dioxide
are produced when 10.0 moles
propane (C3H8) are burned in
excess oxygen?
1. Write & balance the equation
C3H8 + 5 O2  3 CO2 + 4 H2O
2. Find the mole ratio between
propane and carbon dioxide:
Practice problem
How many moles of carbon dioxide are produced
when 10.0 moles propane (C3H8) are burned in
excess oxygen?
1. Write & balance the equation
C3H8 + 5 O2  3 CO2 + 4 H2O
2. Find the mole ratio between propane and carbon dioxide:
1 mol C3H8 : 3 mol CO2
3. Multiply ratio by known amount from
problem
Practice problem
How many moles of carbon dioxide are produced
when 10.0 moles propane (C3H8) are burned in
excess oxygen?
1. Write & balance the equation
C3H8 + 5 O2  3 CO2 + 4 H2O
2. Find the mole ratio between propane and carbon dioxide:
1 mol C3H8 : 3 mol CO2
3. Multiply ratio by known amount from
problem
10 mol C3H8 x 3 mol CO2 = 30 mol CO2
1 mol C3H8
Standard Check
Given the number of moles of a
reactant or product can you solve
for the moles of another reactant
or product?
Second part of standard
Sc2d. Identify and solve different
types of stoichiometry problems,
specifically relating mass to moles
and mass to mass.
Stoichiometric Calculations :
continued
 Moles to mass
 Determine the mass of sodium needed
when sodium reacts with 1.25 moles of
chlorine gas to form sodium chloride
1. Write & balance the equation
2. Find the mole ratio between sodium and
chlorine:
3. Multiply ratio by known amount from
problem
4. Change moles to grams
Stoichiometric Calculations :
continued
 Moles to mass

Determine the mass of sodium needed when sodium
reacts with 1.25 moles of chlorine gas to form sodium
chloride
1. Write & balance the equation
2 Na + Cl2  2 NaCl
2. Find the mole ratio between sodium and chlorine: 2
mol Na: 1 mol Cl2
3. Multiply ratio by known amount from problem
1.25 mol Cl2 x 2 mol Na = 2.50 mol Na
1 mol Cl2
4. Change moles to grams
2.50 mol Na x 22.99 g Na = 57.48 g Na
1 mol Na
Stoichiometric Calculations :
continued
 Mass to mass
 25.0 g ammonium nitrate produces
dinitrogen monoxide and water when it
decomposes. Find the mass of water
produced.
1. Start with a balanced equation
2. Change grams to moles
3. Find the mole ratio
4. Multiply ratio by known amount from
problem
5. Change moles to grams
Stoichiometric Calculations :
continued
 Mass to mass

25.0 g NH4NO3 produces N2O and H2O when it
decomposes. Find the mass of water produced.
1. Start with a balanced equation
NH4NO3  N2O + 2 H2O
2. Change grams to moles
25.0g NH4NO3 x 1 mol NH4NO3 = 0.312 mol NH4NO3
80.04g NH4NO3
3. Find the mole ratio : 1 mol NH4NO3 : 2 mol H2O
4. Multiply ratio by known amount from problem
0.312 mol NH4NO3 x 2 mol H2O = 0.624 mol H2O
1 mol NH4NO3
5. Change moles to grams
0.624 mol H2O x 18.02g H2O = 11.2 g H2O
1 mol H2O
Standard Check
Can you identify and solve
different types of stoichiometry
problems, specifically relating:
 mass to moles
 and mass to mass.
Third part of standard
 Sc2e. Demonstrate the conceptual principle of
limiting reactants.
 What should you be able to do:
 Work the problem and solve for the amount
of product possible
 Use that info to pick out the limiting
reactant
 Use that info to pick out the excess reactant
 Decide how much product is theoretically
made
Limiting Reactants
 Limiting reactant: the reactant that runs out 1st
 Excess reactant: the reactant that is left over
 To solve limiting reactant problems:
1. Work the problem using both reactants and
solve the number of moles produced of the
same product
 The one that produces the least amount of
moles is the limiting reactant
Standard Check
 Can you solve mole to mole conversions?
 Can you solve mass to mole conversions?
 Can you solve mass to mass
conversions?
 Can you solve for theoretical yield and
identify the limiting and excess reactant?
Standard
 SCSh5. Students will demonstrate the
computation and estimation skills
necessary for analyzing data and
developing reasonable scientific
explanations.
 You will calculate percent yield as a way
to analyze lab data and pinpoint reasons
for discrepancies.
Percent Yield
 Theoretical yield is the maximum amount of
product that can be produced (determined
through stoichiometric calculations)
 Actual yield is the amount produced in
lab/from experiment
 Percent yield is the ratio of actual to
theoretical expressed as a percent.
% yield =
actual yield
x 100
theoretical yield
Why dont we have 100%?
Reasons for a decrease in percent
yield
 Loss of products or reactants (spill,
stick to side of containers)
 Reaction doesn’t go 100%
 Not all reactants react
 Impure reactants
 Dirty containers
Percent Yield continued
CH4 + 2 O2  CO2 + 2 H2O
 Determine the % yield if 10.0 grams methane
burned to produce 19.5 g water.
1. Use stoichiometric calculations to
determine theoretical yield of water
produced from 10.0 g methane.
2. Using 19.5 g H2O as the actual yield, solve
for % yield.
Standard Check
 Can you solve mole to mole conversions?
 Can you solve mass to mole conversions?
 Can you solve mass to mass
conversions?
 Can you solve for theoretical yield and
identify the limiting and excess reactant?
 Can you solve for percent yield?
 Can you give reasons why the percent
yield will not be 100%?